Flat plate heat pipes or heat spreaders made of copper with sintered copper internal wicks and water as the working fluid are currently available. Copper-water is the best combination of container and working fluid for heat pipes in the temperature range 20 to 100 C. from a number of perspectives, including toxicity, flammability and performance.
A significant drawback of this copper-water combination is the weight of the heat pipe resulting from the high density of copper (0.34 lb/cubic inch), and its relatively low yield strength (about 10,000 psi). A heat pipe made of copper approximately 8 inchesxc3x9712 inchesxc3x970.25 inch thick of necessity weighs about 3.5 lb.
I have invented a flat plate heat pipe with the benefits of the copper-water combination, which is significantly lighter (about 2 lb. for the size described above), while also being stronger and more durable and less expensive to produce.
My invention employs a copper foil internal container for compatibility with water and a heat treated aluminum outer plate surface to provide structural strength and resistance to puncture.
Flat copper-water heat pipes are currently produced by several methods. One approach is to arrange multiple traditional cylindrical heat pipes in a parallel array soldered to a flat plate. A second layer of heat pipes may be arranged perpendicular to the first layer to achieve high heat flow in all directions, resulting in an isothermal condensing surface.
Another method uses a machined copper container formed by a very shallow pan about 0.2 inch deep with a grid of closely spaced supports remaining after the pan has been machined from a 0.2 inch thick plate of copper. A copper powder wick is sintered into the pan and a fill tube is soldered or welded in place. A copper sheet covering the pan is then welded around the periphery of the resulting heat pipe container, which is checked for leaks before being charged with an appropriate amount of water and sealed by clamping the fill tube and then welding it to permanently hermetically seal the finished flat heat pipe.
The drawbacks of the above-described approaches are the relative vulnerability to external insults resulting when one attempts to achieve a light weight heat pipe. The large flat surfaces of the heat pipe, when made of copper, must be very thin to achieve a reasonably light weight. Internal supports (also solid copper) must be closely spaced to allow the thin copper walls of the heat pipe to support even the external atmospheric pressure (internal pressure of the heat pipe is very low). When this is done, the flat surfaces are quite fragile if bumped by a sharp object. Machining of the copper container pan with support posts requires significant time and cost.
U.S. Pat. No. 5,642,776 describes a very light weight heat pipe with a semi-rigid plastic foam wick. Its purpose is to provide a useful, if low efficiency, electrically insulated heat pipe. Its envelope container is comprised of a very thin and flexible film utilizing several layers of very thin foil and plastic film. No protective outer surface plates are used, rendering the heat pipe very vulnerable to failure by puncture. Also, the heat pipe""s internal vacuum would cause its thin film envelope to partially collapse, significantly impairing performance by reducing its contact area with the object being cooled. It is, therefore, a low performance, puncture vulnerable and somewhat flexible device, whereas the current invention approaches the ultimate in flat heat pipe performance while reducing cost and weight in a very durable and puncture resistant rigid flat heat pipe.
U.S. Pat. No. 6,446,706 B1 describes a very flexible heat pipe which can be made in the form of a tape for application to objects to be cooled. This invention also uses a very thin and flexible multi-layer film as its envelope container, about 0.007 inch thick. Only 0.002 inch is made of very thin metal foil, in two layers of 0.001 inch. These foil layers are for gas leak protection only and do not offer any protection against puncture and the resulting heat pipe failure. There is not, nor could there be, any protective plate applied to the outside of the heat pipe for rigidity and puncture resistance, because this invention""s purpose is to produce a very flexible heat pipe which may be applied as tape to irregular surfaces. The wick materials described are, of necessity, flexible. Only copper felt and fine screen are suggested. The separator materials which provide open vapor space within the heat pipe also need to be flexible, and only coarse screen and coarse copper felt are suggested. Because the flexibility criterion dictated the use of a mostly plastic film as the container, this heat pipe would be a lower performance, puncture prone, very flexible device, whereas the current invention approaches the ultimate in flat heat pipe performance while reducing cost and weight in a very durable and puncture resistant rigid flat heat pipe.
U.S. Pat. No. 6,392,883 describes a flat heat pipe but gives no specific guidance or performance information. The heat pipe discussed is a component of a multi-component heat dissipation system. No laminations or wick details are taught.
A flat plate copper-water heat pipe employs thin copper foil for a container to avoid the large weight penalty of a machined copper container. A sintered copper powder wick with a waffle shaped grid molded into one face provides mechanical support of the foil container while the open space of the waffle grid allows free flow of steam to cool areas of the container surface for condensation heating. Thin heat treated aluminum plates are bonded to both the evaporator and condenser surfaces of the copper foil container with a very thin film of thermally conductive transfer tape, providing strength and durability while preserving high thermal performance.
Accordingly, several objects and advantages of my invention are as follows. The weight of the copper-water heat pipe core is minimized by utilizing thin copper foil to make the container, and by making the sintered copper wick (covering the entire evaporator surface of the flat heat pipe) as thin as is practical.
The space between the wick and the condensing copper foil surface is kept partially open to steam flow, both perpendicularly to the flat plate surface and laterally, by any of several means. Three such means are 1) a grid molded into the sintered copper wick, 2) a copper screen, and 3) a flat sheet of rigid copper open cell foam. Any of these serve to allow free flow of steam to any cool area of the heat pipe condensing surface, keeping the condensing surface essentially isothermal even when the cooling load does not coincide with the heated area of the evaporator surface.
The heat pipe so constructed would not remain flat or be structurally stable and durable without the addition of other elements for strength, stability and durability. This invention answers this requirement by laminating a much lighter and stronger plate of heat treated aluminum to both faces of the flat heat pipe. The resulting completed flat heat pipe structure is reliably flat, much less fragile to damage by sharp objects, and most importantly weighs little more than half the weight of a similar all-copper flat plate heat pipe.